This invention is directed to apparatus for monitoring and controlling the deposit of materials on substrates by vapor deposition. The invention is more particularly concerned with quartz crystal deposition monitors, in which the resonance frequency of a piezoelectric crystal is monitored, with changes in the resonance frequency corresponding to changes in thickness of the deposited material. More specifically, the invention relates to a multiple-crystal detector, in which a number of crystals can be moved in turn into a monitoring station, so that the monitoring and control function can continue after one of the crystals loses its ability to detect further thickness changes.
Quartz crystal monitors have been widely used to monitor vacuum deposition processes and to control accurately the amount of material deposited and the rate of deposit onto a surface. In these monitors, one or more piezoelectric crystals which can be quartz, barium titanate, or another suitable material, are connected into a resonance circuit so that the natural resonant frequency of one crystal can be monitored. The natural resonance is primarily dependent upon total mass and geometry of the crystal, and the resonance frequency drops in relation to the amount of material coated onto it during a vacuum deposition process. However, as the deposited material builds up, the sharpness of resonance diminishes, and eventually a point is reached in which the crystal can no longer monitor the process accurately or effectively. Then, the quartz crystal must be replaced.
A number of multi-crystal quartz oscillator deposition monitors have been proposed in which there are a number of crystals that can be rotated in turn into a monitoring position to facilitate changing of crystals with a minimum of down time and without introducing mechanical stresses from insertion of the crystals. Such multi-crystal monitors are described, for example in U.S. Pat. No. 4,362,125 and U.S. Pat. No. 3,383,238.
Mechanical and thermal stresses can also affect the resonance frequencies of the crystals in the monitor. However, prior multi-crystal monitors have not incorporated heat shields or heat sinks, and are subject to temperature stresses from the heat produced in a vacuum deposition process.